Add task for signing packages after they've been staged

This is a piece of the jenkins chain. This task allows us to take locally
staged packages and ship them to the distribution server (known to have tools
for all kinds of signing), sign them, and bring them back. The twist is, we
don't want to duplicate all the hacky logic already in the packaging repo to
do this, so we send the packaging repo along with it, via the remote
bootstrap workflow already used in the remote tasks. Signed packages are
then retrieved and ready to go. This is actually an unfortunate duplication of
the logic of pl:remote, but it is necessary because we have to deviate
from that workflow because we're handling files that aren't in git - everything
under pkg.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>